Method for preparing tantalum or niobium powders used for manufacturing capacitors

ABSTRACT

Disclosed relates to a method for preparing tantalum or niobium powders used for manufacturing capacitors in an electrolytic reducing reactor including an anode, a cathode and a molten salt, the method comprising: obtaining a tantalum or niobium oxide, expressed by Ta 2 O (5-y)  or Nb 2 O (5-y)  where y=2.5 to 4.5, from a tantalum pentoxide Ta 2 O 5  or a niobium pentoxide Nb 2 O 5  generated partially by an alkaline metal electrolytically reduced via a first electrolytic reducing reaction that reduces an alkaline metal oxide from a molten salt comprising at least one metal halogen compound, selected from the group consisting of alkaline metal and alkaline earth metal, and an alkaline metal oxide on the cathode; and preparing a tantalum or niobium powder by a first electrolytic reducing reaction that reduces at least one metal halogen compound selected from the group consisting of the alkaline metal oxide and the alkaline earth metal on the cathode and by a second reducing reaction with the tantalum or niobium oxide, represented by Ta 2 O (5-y)  or Nb 2 O (5-y)  where y=2.5 to 4.5.

BACKGOUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for preparing tantalum or niobium powders used for manufacturing capacitors and, more particularly, to a method for preparing tantalum or niobium powders in high purity for manufacturing capacitors of high capacity from tantalum pentoxides Ta₂O₅ or niobium pentoxides Nb₂O₅ via a molten salt electrolytic reduction method.

2. Description of Related Art

Conventional methods for preparing tantalum powders include a method of reducing Ta₂O₅ via carbon, a method of reducing Ta₂O₅ via calcium and magnesium, a method of reducing TaCl₅ via hydrogen and a method of reducing K₂TaF₇ via a molten salt electrolysis or via sodium. According to the carbon reduction method, the amount of remaining oxygen is very large. In case of calcium and magnesium reduction method, it is difficult to control the reaction. Besides, in case of the hydrogen reduction method, it is difficult to control the grain sizes. Furthermore, the method of reducing K₂TaF₇ via the molten salt electrolysis prepares the powders in the form of dendrite, which results in a limitation on use.

Alternatively, a method of reducing K₂TaF₇ via Na has been applied in the process for preparing tantalum powders for capacitors used in industries. However, this method still has some drawbacks that the yield is decreased due to unreacted reactants generated by applying a large amount of diluent for decreasing the temperature of reaction so as to prepare tantalum powders for manufacturing capacitors of high capacity and it is known that it is impossible to prepare the powders of 100,000 CV or more.

Accordingly, methods of controlling reduction processes of Ta₂O₅ via Mg have been attempted to prepare tantalum powders used for manufacturing capacitors of high capacity. There have been disclosed methods of reducing heavy metal oxides via alkaline earth metals in U.S. Patent Application Publication Nos. 2003/0070509 and 2003/0110890, and in U.S. Pat. Nos. 6,136,062 and 6,171,363. The most serious problem in those methods is that they are accompanied with strong exothermic reactions during reductions. When the oxides mixed stoichiometrically to be reduced and the reduction mixture are subjected to react, the reaction proceeds in an instant and the reaction temperature is raised to 1,000° C. or more. To overcome this problem, as disclosed in U.S. Pat. No. 6,136,062, in the process of reducing tantalum and/or niobium oxides via magnesium powders, the first reduction stage is carried out as far as an average composition corresponding to (Nb,Ta)O_(x) where x=0.5 to 1.5 and the oxides of the reduction agent such as MgO and excessive Mg powders are removed and, then, the second reduction stage is carried out to remove residual oxygen. According to these methods, it is possible to synthesize minute tantalum and niobium powders by preventing such rapid increase of the reaction temperature, however, they still have some drawbacks that it is difficult to solve the problem of carbon contamination; they require a large amount of reducing agent and acid when leaching; and the reactions are made in two stages, which deteriorates their productivities.

Meanwhile, a method of reducing Ta₂O₅ in a molten salt via the following reaction has been developed recently:

Ta₂O₅+10Na+5MgCl₂→2Ta+5MgO+10NaCl  (1)

When using the above method, it is possible to prepare tantalum powders having a capacity of 100,000 CV or more, however, it has still drawbacks that it requires a large amount of diluent in order to reduce the reaction heat generated during the reaction between Mg and Ta₂O₅ reduced via Na; the residual oxygen content in tantalum and niobium after reaction is very high, about 1 to 3.7%; and the final powders are dispersed in the molten salt, which causes difficulties in collecting the final powders.

Moreover, some researches have been reported as disclosed in International Publication No. WO99/64638 and in U.S. Pat. No. 6,540,903 for preparing metals by carrying out direct electrolytic reductions for metal oxides in a molten salt. However, in order to electrolyze metal oxides directly, it is necessary to set electric potential of the electrolytic reduction lower than the decomposition potential of the molten salt used as a reaction medium and higher than the reduction potential of the metal oxides. Since the applied voltage cannot be increased, the reaction of electrolytic reduction proceeds slowly, which decreases productivity. Besides, since the method aims at converting the metals themselves, it is not suitable for preparing tantalum and niobium powders used for manufacturing capacitors.

To apply the tantalum and niobium powders to capacitors, it is necessary to establish open pores over a certain size for impregnating uniformly into sintered pellets in the stages of the anode oxidization and filling cathode material. For this sake, it is required that primary grains having appropriate mesh structures be formed via local sinterings between initial grains reduced in the stage of metal reduction, however, it is difficult to expect the local sinterings between the reduced metal powders, since the oxygen atoms in the metal oxide are ionized directly to separate in case of the conventional direct electrolytic reduction process. Moreover, in case of the conventional indirect electrolytic reduction process using alkaline earth metals, it is impossible to form mesh grains since heat of reaction in the adiabatic process is not generated during the direct reducing reaction with heavy metal oxides.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been invented to is solve the problems described above, an object of the present invention is to decrease the addition of alkaline earth metal halogen compound and to provide a method for preparing tantalum or niobium powders having mesh structures suitable for manufacturing capacitors by inducing local sinterings of tantalum or niobium powders via heat of combustion reaction in the adiabatic process generated during the indirect reducing reaction with alkaline earth metals

To accomplish the objects of the present invention, there is provided a method for preparing tantalum or niobium powders used for manufacturing capacitors in an electrolytic reducing reactor including an anode, a cathode and a molten salt, the method comprising:

obtaining a tantalum or niobium oxide, expressed by Ta₂O_((5-y)) or Nb₂O_((5-y)) where y=2.5 to 4.5, from a tantalum pentoxide Ta₂O₅ or a niobium pentoxide Nb₂O₅ generated partially by an alkaline metal electrolytically reduced via a first electrolytic reducing reaction that reduces an alkaline metal oxide from a molten salt comprising at least one metal halogen compound, selected from the group consisting of alkaline metal and alkaline earth metal, and an alkaline metal oxide on the cathode; and

preparing a tantalum or niobium powder by a first electrolytic reducing reaction that reduces at least one metal halogen compound selected from the group consisting of the alkaline metal oxide and the alkaline earth metal on the cathode and by a second reducing reaction with the tantalum or niobium oxide, represented by Ta₂O_((5-y)) or Nb₂O_((5-y)) where y=2.5 to 4.5.

Furthermore, the present invention provides a method for preparing tantalum or niobium powders used for manufacturing capacitors in an electrolytic reducing reactor, wherein the molten salt comprises of an alkaline metal halogen compound and an alkaline earth metal halogen compound; the alkaline earth metal halogen compound is first electrolytically reduced on the cathode; the tantalum pentoxide Ta₂O₅ or the niobium pentoxide Nb₂O₅ is indirectly reduced by the reduced alkaline earth metal; and local sinterings are induced by potential controls.

Moreover, the present invention provide a method for preparing tantalum or niobium powders used for manufacturing capacitors further comprises leaching and washing the tantalum or niobium powders obtained during the second reducing reaction in an inorganic acid. After the step of leaching and washing in the inorganic acid, the tantalum or niobium powders have mesh structures.

The tantalum pentoxides Ta₂O₅ or the niobium pentoxides Nb₂O₅ may be used in the form of powder or porous pellet. If the tantalum pentoxides Ta₂O₅ or the niobium pentoxides Nb₂O₅ are used in the form of porous pellet, for example, it is possible to form perforations in the porous pellet so as to ensure a smooth transfer path of the molten salt into the porous pellet. The porous pellet of the tantalum pentoxides Ta₂O₅ or the niobium pentoxides Nb₂O₅ is produced by molding source material powders via a sintering process for providing a proper solidity. Accordingly, the perforations are made in the molded pellet prior to the sintering process. To obtain metal tantalum or niobium powders suitable for manufacturing capacitors, it is desirable to use the tantalum pentoxides Ta₂O₅ or the niobium pentoxides Nb₂O₅ having a content of metal impurities of 150 ppm or less and a carbon content of 50 ppm or less.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram showing a rough configuration of a molten salt electrolytic reducing reactor for carrying out a method for preparing tantalum or niobium powders used for manufacturing capacitors in accordance with the present invention, wherein a cathode basket is used for charging the reactor with Ta₂O₅ or Nb₂O₅;

FIG. 2 depicts a molten salt electrolytic reducing reactor, where Ta₂O₅ or Nb₂O₅ pellet is charged on the bottom of the reactor, instead of the cathode basket of FIG. 1;

FIG. 3 depicts a molten salt electrolytic reducing reactor, where the Ta₂O₅ or Nb₂O₅ pellet is charged in the molten salt, instead of the cathode basket of FIG. 1;

FIG. 4 is a schematic diagram illustrating a rough configuration of a cathode basket used in a molten salt electrolytic reducing reactor for carrying out the method for preparing tantalum or niobium powders used for manufacturing capacitors in accordance with the present invention;

FIG. 5 is a schematic diagram depicting a rough configuration of the Ta₂O₅ or Nb₂O₅ pellet used in the molten salt electrolytic reducing reactor for carrying out the method for preparing tantalum or niobium powders used for manufacturing capacitors in accordance with the present invention;

FIG. 6 is a schematic diagram showing a rough configuration of the cathode used in the molten salt electrolytic reducing reactor for carrying out the method for preparing tantalum or niobium powders used for manufacturing capacitors in accordance with the present invention;

FIG. 7 shows a preparation process of tantalum or niobium powders used for manufacturing capacitors in accordance with a preferred embodiment of the present invention; and

FIG. 8 depicts shapes of the niobium powders prepared in accordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, a detail description of the present invention will be given with reference to the attached drawings. The present invention is not restricted to the following embodiments, and many variations are possible within the spirit and scope of the present invention. The embodiments of the present invention are provided in order to more completely explain the present invention to anyone skilled in the art.

The present invention decreases the addition of alkaline earth metal halogen compound sharply by causing a partial reducing reaction via alkaline metals, with the simultaneous addition of alkaline earth metal halogen compound such as magnesium or calcium, besides alkaline metal oxides. Moreover, the present invention prepares tantalum or niobium metal powders having mesh structures suitable for manufacturing capacitors by inducing local sinterings of tantalum or niobium powders via heat of combustion reaction in the adiabatic process generated during the indirect reducing reaction with the alkaline earth metals.

Since the reduction potentials of magnesium chloride and calcium chloride at 650° C. is −2.57 V and −3.43 V, respectively, and the reduction potential of lithium oxide, a precursor of lithium as a main reducing agent, is −2.46 V, the reduction potentials for the formers have a negative value greater than that of the later. Accordingly, the first reducing reaction proceeds in such a manner that a partial reducing reaction of the tantalum pentoxides Ta₂O₅ or the niobium pentoxides Nb₂O₅ proceeds due to the indirect reducing reaction via the reduction of the lithium oxides added in a low stoichiometric proportion and, then, a complete reducing reaction is made, after the reduction of the lithium oxides, via the magnesium chloride and the calcium chloride when reaching the reduction potential of the magnesium chloride or the calcium chloride, thus forming tantalum or niobium metal powders having mesh structures.

The temperatures of the adiabatic reaction at 650° C. during the reducing reactions of tantalum pentoxides Ta₂O₅ and niobium pentoxides Nb₂O₅ via lithium, magnesium and calcium are as follows:

Ta₂O₅+10Li→2Ta+5Li₂O, T_(ad(650° C.)): 25° C.  (2)

Ta₂O₅+5Mg→2Ta+5MgO, T_(ad(650° C.)): 2,326° C.  (3)

Ta₂O₅+5Ca→2Ta+5CaO, T_(ad(650° C.)): 3,294° C.  (4)

Nb₂O₅+10Li→2Nb+5Li₂O, T_(ad(650° C.)): 25° C.  (5)

Nb₂O₅+5Mg→2Nb+5MgO, T_(ad(650° C.)): 2,393° C.  (6)

Nb₂O₅+5Ca→2Nb+5CaO, T_(ad(650° C.)): 3,101° C.  (7)

It is difficult to expect the local sinterings of the first grain of tantalum or niobium during the indirect reduction via lithium; however, it is possible to control the temperature of the second reducing reaction by regulating the initial addition of lithium oxides. That is, when the amount of residual tantalum oxides or niobium oxides is excessive or the precipitation rate of Mg or Ca is too high, over-fusion and over-growth of metal powders may occur due to the over-heating. Accordingly, it is necessary to control the molar ratio of addition and the precipitation rate cautiously.

In accordance with the present invention, it is possible to prepare tantalum or niobium metal powders suitable for manufacturing capacitors by electrolytically reducing the alkaline metal oxide and the alkaline earth metal halogen compound, which are components of the molten salt, and by reacting the electrolytically reduced alkaline metals and alkaline earth metals with Ta₂O₅ or Nb₂O₅ in the form of powder or porous pellet in the cathode part of the electrolysis device.

FIG. 1 is a schematic diagram showing a rough configuration of a molten salt electrolytic reducing reactor 100 for carrying out a method for preparing tantalum or niobium powders used for manufacturing capacitors in accordance with the present invention, wherein a cathode basket is used for charging the reactor with Ta₂O₅ or Nb₂O₅; FIG. 2 depicts a molten salt electrolytic reducing reactor 200, where Ta₂O₅ or Nb₂O₅ pellet is charged on the bottom of the reactor, instead of the cathode basket of FIG. 1; and FIG. 3 depicts a molten salt electrolytic reducing reactor 300, where the Ta₂O₅ or Nb₂O₅ pellet is charged in the molten salt, instead of the cathode basket of FIG. 1.

Referring to FIG. 1, a molten salt electrolytic reducing reactor 100 comprises an electric furnace 1, a reaction vessel 3, an anode 5, a cathode 6 and other peripheral devices. The molten salt electrolytic reducing reactor 100 comprises an insulator 2 insulating the reaction vessel 3 from the outside, which produces an inert gas atmosphere in a chamber, and may have any structures that can control the reaction temperature within 600° C. to 1,000° C. and a current supplier 9 to be operatated independently.

A thermocouple 10 is provided in the reaction vessel 3 of the molten salt electrolytic reducing reactor 100. The thermocouple 10 measures temperature variations of source material during the reducing reaction in order to identify a start point of reaction and is coupled to a computer via a data collector so as to prevent over-heating. A vacuum pump 8 for producing a vacuum is connected to an upper end 11 of the reactor chamber. The vacuum pump 8 deflates the reactor chamber to produce a vacuum and supplies an inert gas into the reactor chamber via a separate valve coupled. The reactor chamber may be maintained in a vacuum, under atmospheric pressure or under 10 atm or less.

The anode 5 may be composed of platinum or pyro-carbon that can endure the corrosive environment, which may be induced due to oxygen generated during the electrolytic reducing reaction and, preferably, Fe₃O₄ as a high-conductive ceramic material may be used.

The source material of Ta₂O₅ or Nb₂O₅ is charged in the form of powder in a cathode basket of the cathode 6 of the reaction vessel 3.

Meanwhile, the source material of Ta₂O₅ or Nb₂O₅ may be used in the form of porous pellet after a sintering process for molding the source material powders and providing a proper solidity, thus increasing productivity. In this case where Ta₂O₅ or Nb₂O₅ pellet is used, the reaction vessel 3 can be used as a cathode and pellets 16 are positioned on the bottom of the reaction vessel 3 as shown in FIG. 2. Here, a plurality of anodes may be provided for applying a current uniformly. Referring to FIG. 3, it is possible to arrange a plurality of pellets 26 connected in series to each other and floated in the molten salt so as to promote the reducing reaction.

In cases where the cathode basket and the molded pellet are applied, it is possible to separate the product readily from the molten salt and reuse the molten salt after terminating the reducing reaction, thus increasing productivity and economical efficiency.

FIG. 4 is a schematic diagram illustrating a rough configuration of a cathode basket used in a molten salt electrolytic reducing reactor for carrying out the method for preparing tantalum or niobium powders used for manufacturing capacitors in accordance with the present invention; FIG. 5 is a schematic diagram depicting a rough configuration of the Ta₂O₅ or Nb₂O₅ pellet used in the molten salt electrolytic reducing reactor for carrying out the method for preparing tantalum or niobium powders used for manufacturing capacitors in accordance with the present invention; and FIG. 6 is a schematic diagram showing a rough configuration of the cathode used in the molten salt electrolytic reducing reactor for carrying out the method for preparing tantalum or niobium powders used for manufacturing capacitors in accordance with the present invention;

With reference to FIG. 4, a cathode basket 400 comprises a porous ceramic vessel 41, an inner electrode 42 and an outer electrode 43. If the source material is used in the form of powder, the porous ceramic vessel 41 can prevent the powders from being dispersed into the molten salt. The porous ceramic vessel 41 includes a bolt 44 for coupling electrodes. When charging the porous ceramic vessel 41 with Ta₂O₅ or Nb₂O₅ powders, steel wool or steel mesh 45 can be mixed to facilitate applying a current from the inner electrode 42 to the source material.

Referring to FIG. 5 depicting a source material pellet 51 used instead of the cathode basket, a plurality of perforations 52 are made in the molded pellet prior to a sintering process to make the molten salt penetrate readily into the pellet 51. The space and the number of the perforations may be set based on the density of the molded pellet and a large number of perforations are required in a high-dense molded pellet.

Now referring to FIG. 6, a single type cathode 600 comprises a high-conductive conductor 65 and a non-conductive ceramic filter 63 to be charged with Ta₂O₅ or Nb₂O₅ powders 64. The single type cathode 600 includes a source material powder inlet 61 on the top thereof, coupled to an outer conduit 62, which connected to the non-conductive ceramic filter 63. Accordingly, the Ta₂O₅ or Nb₂O₅ powders 64 injected through the inlet 61 are charged in the non-conductive ceramic filter 63. The outer conduit 62 performs a function of making airtight for charging the non-conductive ceramic filter 63 with the Ta₂O₅ or Nb₂O₅ powders 64, not scattered.

It is desirable that the non-conductive ceramic filter 63 has a porosity of 10 to 60% so that the Ta₂O₅ or Nb₂O₅ powders 64 transfer to the molten salt smoothly. The non-conductive ceramic filter 63 may be composed of magnesia, alumina, magnesium aluminate, zirconia and the like.

A high-conductive conductor 65 is arranged in the outer conduit 62 extended to the inside of the non-conductive ceramic filter 63. The high-conductive conductor 65 functions as an electrode connection portion contacting with the Ta₂O₅ or Nb₂O₅ powders 64 and is made of SUS, copper, titanium, tantalum and the like.

FIG. 7 shows a preparation process of tantalum or niobium powders used for manufacturing capacitors in accordance with a preferred embodiment of the present invention.

With reference to FIG. 7, the preparation process comprises: preliminarily treating the Ta₂O₅ or Nb₂O₅ powders to be readily reduced electrochemically [S10]; charging the reducing reactor with a molten salt selected for electrochemical reduction of the Ta₂O₅ or Nb₂O₅ and the source material preliminarily treated [S11]; elevating the temperature of the reactor and keeping the elevated temperature [S12]; performing an electrolytic reduction by applying a current to control the velocity of the reaction of the source material and a reducing agent [S13]; extracting the resultant product generated in ST 13 [S14]; cooling the molten salt [S15]; milling the resultant product [S16]; leaching and washing impurities contained the resultant product [S17]; drying in a vacuum [S18]; classifying [S19]; agglomerating and annealing [S20]; deoxidizing and passivating [S21]; leaching and washing [S22]; and drying and packing in a vacuum [S23].

The molten salt in accordance with the present invention comprises at least one metal halogen compound, selected from the group consisting of alkaline metal, such as Li, Na, K, etc., and alkaline earth metal, such as Mg, Ca, etc., and an alkaline metal oxide. As the halogen compound, alkaline metal halogen compound, alkaline earth metal halogen compound, eutectic halogen compound such as Li—K, Li—Ca, Li—Mg, etc. may be used and, moreover, any combinations and compositions thereof are applicable. Here, the alkaline metal oxide is added to the extent of being dissolved by the corresponding salt and used as the molten salt.

The reducing reaction of tantalum pentoxide Ta₂O₅ or niobium pentoxide Nb₂O₅ in the molten salt proceeds in the following steps.

In step 1, an alkaline metal ion in the molten salt is reduced on cathode;

M⁺+e⁻

M  (8)

wherein M denotes an alkaline metal such as Li, Na, K, etc.

In step 2, a reaction occurs via the alkaline metals on the surface of Ta₂O₅ or Nb₂O₅ positioned on cathode;

Ta₂O₅ (or Nb₂O₅)+2yM→Ta₂O_((5-y))(or Nb₂O_((5-y)))+yM₂O  (9)

In step 3, an alkaline earth metal ion in the molten salt is reduced on cathode;

M_(AE) ²⁺+2⁻

M_(AE)  (10)

-   -   wherein M_(AE) represents an alkaline earth metal such as Mg,         Ca, etc.

In step 4, a reducing reaction proceeds via the alkaline earth metals on the surface of Ta₂O_((5-y))(or Nb₂O_((5-y))) positioned on cathode;

Ta₂O_((5-y))(or Nb₂O_((5-y)))+(5-y)M_(AE)→2Ta(or Nb)+(5-y)M_(AE)O_((5-y))  (11)

To progress the reducing reaction more smoothly and to prevent local over-heating in the molten salt, the value of y is regulated in the range of 2.5 to 3.5 and, particularly, it is desired to stir the molten salt so as to promote the re-oxidized alkaline metal oxides to be re-dissolved to the molten salt.

One of the advantages of the present invention is to control the reaction temperature by regulating the content of alkaline metal oxides and the applied current. That is, if regulating the time of applying a current on a specific reduction potential that the alkaline metal ions or the alkaline earth metal ions as a composition of the molten salt are to be reduced, the amount of metals to be reduced by reacting with the source material oxides can be controlled. More particularly, if a rapid increase of temperature is detected by observing the temperatures of the reduction in real time from the thermocouple buried in the molten salt, the current being applied may be cut off or lowered to operate as desired.

If a higher temperature of the reducing reaction is allowed, the reducing reaction of the source material oxides via the alkaline metals or the alkaline earth metals may be made rapidly and completely in theory, however, it may be difficult to prepare minute tantalum or niobium powders due to sudden grain growths.

In the preparation process of the present invention, it is desirable to use tantalum pentoxides Ta₂O₅ or niobium pentoxides Nb₂O₅ in high purity as a starting material and a molten salt in which alkaline metal oxides and alkaline earth metal oxides are dissolved. Metal tantalum and niobium powders most suitable for manufacturing capacitors can be obtained when the total content of metal impurities in the source material oxide is 150 ppm or less. Accordingly, it is desirable to use source material oxide powders, alkaline metal oxides and alkaline earth metal halogen compounds, each having an impurity content of 150 ppm or less, however, a portion of metal impurities and nonmetallic impurities may be removed during the electrolytic reducing reaction and the processes of leaching and vacuum annealing. But, it is desirable to use tantalum pentoxides and niobium pentoxides having a carbon content of below 50 ppm or less, preferably, 10 ppm or less.

In accordance with the method for preparing tantalum or niobium powders used for manufacturing capacitors of the present invention, tantalum pentoxides Ta₂O₅ or niobium pentoxides Nb₂O₅ are added to an alkaline metal or alkaline earth metal (one of Li, Na, K, Mg and Ca, or their mixture) halogen molten salt, in which alkaline metal oxides (one of Li, Na, K, Mg and Ca, or their mixture) is dissolved; a current is applied to the halogen molten salt to reduce the alkaline metals electrochemically; the reduced alkaline metals reduce a portion of Ta₂O₅ or Nb₂O₅; after terminating the reduction of the alkaline metal oxides, a second reduction of the alkaline earth metals is started to complete the chemical reduction of unreacted tantalum or niobium oxides, thus obtaining Ta or Nb powders in high purity and alkaline metal and alkaline earth metal oxides; and, then, those products are leached and washed via sulphuric acid H₂SO₄ of about 20% to remove the alkaline metal and alkaline earth metal oxides, thereby preparing tantalum or niobium powders in high purity.

The molten salt electrolytic reduction method used in the present invention is an electrochemical reduction process comprises a first reducing reaction that reduces alkaline oxides to alkaline metals under the conditions regulated by alkaline metal salts using a theory that a general heavy metal oxide is reduced via alkaline metals; and a second reducing reaction that is made by reacting the reduced alkaline metals with the heavy metal oxide.

The Ta₂O₅ or Nb₂O₅ powders as a source material may have a grain size from 0.05 μm to 100 μm variously according to its final object. The initial grain size of the source material powders is a variable that has a considerable effect on the electrostatic capacity of the end tantalum and niobium powders. In case of the cathode basket, the source material can be used in the form of powder without any separate processes. However, if the source material is used in the form of pellet, molding and sintering processes are required. For molding the source material powders, Ta₂O₅ or Nb₂O₅ powders are molded to have a porosity of 40 to 90% and perforations are formed in the molded pellet if necessary, then, the molded pellet is sintered at 500° C. to 1,500° C. for 5 minutes to 48 hours. If the sintering temperature is low, the sintering time is maintained long, whereas, if the temperature is high, the time is set to be short. Meanwhile, it is possible to mill an alkaline earth metal salt and mix the milled salt with the source material powders so as to facilitate the formation of porosities.

When selecting the molten salt, it is necessary to consider melting points of the respective alkaline and alkaline earth metal compounds. That is, the melting points of LiCl, Nacl, KCl, MgCl₂ and CaCl₂ are 613° C., 804° C., 773° C., 712° C. and 772° C., respectively. In particular, since the eutectic point in composing LiCl—KCl is lowered to 425° C., it is possible to prepare more minute tantalum and niobium powders at a lower temperature, which may, however, increase the amount of residual oxygen. Accordingly, it is required to set up appropriate operation conditions.

The source material and the salt prepared in such manners are charged in the reaction vessel 3 depicted in FIGS. 1 to 3. Then the vacuum pump 8 operates to deflate the reactor chamber to produce a vacuum and supplies an inert gas into the reactor chamber. Subsequently, heat is applied to the electric furnace 1 to elevate the temperature of the furnace 1 from 500° C. to 1,000° C. according to the kind of the molten salt applied to and to keep the elevated temperature.

The reducing reaction is started when applying a current to the anode and the cathode charged in the molten salt. The capacity of the direct current power supplier for applying a current is varied based on the areas that the anode and the cathode contact with the molten salt and it may have a range of reduction potential of the alkaline metal and alkaline earth metal oxide of −3 V or more, and a current density of 100 mA/cm² or more.

The indirect reducing reaction applying the above formulas (10) and (11) proceeds with a first reduction reaction that reduces alkaline earth metals directly on cathode in the molten salt by adding an alkaline earth metal halogen compound without the addition of an alkaline metal oxide and, then, causes a second reduction reaction of Ta₂O₅ or Nb₂O₅. Here, it is necessary to regulate the precipitation rate of the alkaline earth metals by regulating the electric voltage to control explosive heat of exothermic reaction. To minimize the exothermic reaction and the generation of chlorine gas in quantities, the following sequential indirect reduction method is applied.

If the value of y in formula (9) is set to 3, 1 kg of LiCl, 20.3 g of Li₂O, 45 g of MgCl₂ or 55 g of CaCl₂, and 100 g of Ta₂O₅ or Nb₂O₅ are heated at 650° C.; a current is applied in order to keep a voltage of −2.5 V to −3.0 V, over the decomposition potential of Li₂O, for two hours; and a current is applied in order to keep a voltage of −2.8 V to −3.0 V, over the decomposition potential of MgCl₂ or CaCl₂ for three hours, then, the cathode basket or the pellet is extracted from the molten salt. Here, Ta₂O₅ or Nb₂O₅ may be charged in the molten salt before cooling the molten salt to proceed with a semicontinuous process, whereas, the molten salt is cooled after extracting the product, in case of a batch process. The extracted product is cooled at room temperature and coarsely crushed. Subsequently, the cooled and crushed product is subjected to leaching in an inorganic acid of 20%, particularly, in a sulphuric acid H₂SO₄ solution by applying heat of 40° C. to 80° C. thereto. Furthermore, hydrogen peroxide H₂O₂ solution of 1% to 10% is added to prevent tantalum and niobium powders from reacting with hydrogen.

The leaching process may be performed several times till metal ion impurities are completely removed. The powders passing the leaching process are washed to be neutralized with purified water heated to 40° C. to 80° C. and vacuum dried. The tantalum and niobium powders used for manufacturing capacitors are processed to form agglomerations within a range of 20 μm to 300 μm for enhancing solidity and compactability. The size of the agglomeration may be adjusted in milling or classifying process.

The agglomerated tantalum or niobium powders are charged in a tantalum vessel to be processed by an annealing at 900° C. to 1,500° C. for 30 minutes to two hours based on the first grain sizes under a vacuum level of 10⁻⁵ torr or more in a vacuum sintering furnace. The annealed powders are cooled at room temperature and passivated with an Ar-1% O₂ mixed gas for more than two hours. Subsequently, the passivated powders pulled out from the furnace are subjected to a deoxidizing process at 800° C. to 1,000° C. for two to four hours after mixing with 325-mesh or less of Mg powders 1% to 5% uniformly to remove residual oxygen in the powders. Then, the deoxidized powders are vacuum dried and packed immediately to be in keeping.

Embodiment 1

Tantalum pentoxides Ta₂O₅ having a content of metal impurities of 150 ppm or less and a carbon content of 10 ppm or less were used. The molten salt electrolytic reducing reactor of FIG. 1 was charged with 1 kg of LiCl and 200 g of MgCl₂. 100 g of Ta₂O₅ having an average grain size of 0.3 μm together with steel wool were put into the cathode basket and heated at 700° C. A current was applied in order to keep a voltage of −2.8 V to −3.0 V, over the decomposition potential of MgCl₂ for five hours. Here, temperature variations of the thermocouple inserted therein were observed to determine to change the voltage. That is, if a rapid increase of the temperature was observed, the voltage was decreased. After terminating the reaction, the resultant product was separated from the molten salt so as to facilitate the extraction of the product after the molten salt was cooled. The extracted product was cooled at room temperature and coarsely crushed so as to shorten the time required for the steps of leaching and washing. During the leaching step, an inorganic acid of 20%, particularly, a sulphuric acid H₂SO₄ solution was added and applied heat of 40° C. to 80° C. thereto. Subsequently, hydrogen peroxide H₂O₂ solution of 1% to 10% was added to prevent the tantalum metal powders from reacting with hydrogen.

The tantalum powders used for electrolytic capacitors were doped with a sintering inhibitor such as nitrogen, phosphorus, boron, sulfur, etc., to prevent over-sintering in the step of sintering. In the present invention, the sintering inhibitor in the form of NH₄H₂PO₄ was mixed with phosphorus to be 100 ppm in proportion to the content of tantalum powders and added to the tantalum powders prior to the processes of agglomerating and annealing.

To handle the powders with ease, the tantalum powders doped with phosphorus were classified into 150-mesh and agglomerated. The agglomerated tantalum powders were charged in the tantalum vessel to be processed by an annealing at 1,250° C. for 30 minutes under a vacuum level of 10⁻⁵ torr or more in the vacuum sintering furnace. The annealed powders were cooled at room temperature and passivated with an Ar-1% O₂ mixed gas for more than two hours. Subsequently, the passivated powders pulled out from the furnace were subjected to a deoxidizing process at 900° C. for four hours after mixing with 325-mesh or less of Mg powders 1% to 5% uniformly to remove residual oxygen in the powders. Then, the deoxidized powders were subjected to a leaching process by adding sulphuric acid H₂SO₄ solution at 40° C. to 80° C. thereto till Mg ions were removed. Subsequently, hydrogen peroxide H₂O₂ solution of 1% to 10% was added to prevent the tantalum powders from reacting with hydrogen. The powders passing the leaching process were washed to be neutralized with purified water heated to 40° C. to 80° C. and vacuum dried.

To carry out the measurement of electrostatic capacitor, the dried powders were molded with a tantalum wire in a rectangular mold to have a density of 4.5 to 5.0 g/cm³. The molded pellet was sintered using the vacuum sintering furnace at 1,200° C. for 30 minutes. The sintered pellet was transformed in 0.1 vol % of phosphorus H₃PO₄ solution heated to 60° C. by applying a direct voltage of 40 V to form an amorphous tantalum oxide layer. The results of measuring impurity contents and electric characteristics for the tantalum powders prepared were shown in Tables 1 and 2 below.

Embodiment 2

Tantalum pentoxides Ta₂O₅ having a content of metal impurities of 150 ppm or less and a carbon content of 10 ppm or less were used. The molten salt electrolytic reducing reactor of FIG. 1 was charged with 1 kg of LiCl, 20.3 g of Li₂O and 45 g of MgCl₂. 100 g of Ta₂O₅ having an average grain size of 0.3 μm together with steel wool were put into the cathode basket and heated at 650° C. A current was applied in order to keep a voltage of −2.5 V to −3.0 V, over the decomposition potential of Li₂O for two hours. And, a current was applied in order to keep a voltage of −2.8 V to −3.0 V, over the decomposition potential of MgCl₂ for three hours. Here, temperature variations of the thermocouple inserted therein were observed to determine to change the voltage. That is, if a rapid increase of the temperature was observed, the voltage was decreased. After terminating the reaction, the resultant product was separated from the molten salt so as to facilitate the extraction of the product after the molten salt was cooled. The extracted product was cooled at room temperature and coarsely crushed so as to shorten the time required for the steps of leaching and washing. During the leaching step, an inorganic acid of 20%, particularly, a sulphuric acid H₂SO₄ solution was added and applied heat of 40° C. to 80° C. thereto. Subsequently, hydrogen peroxide H₂O₂ solution of 1% to 10% was added to prevent the tantalum metal powders from reacting with hydrogen.

The tantalum powders for electrolytic capacitors were doped with a sintering inhibitor such as nitrogen, phosphorus, boron, sulfur, etc., to prevent over-sintering in the step of sintering. In the present invention, the sintering inhibitor in the form of NH₄H₂PO₄ was mixed with phosphorus to be 100 ppm in proportion to the content of tantalum powders and added to the tantalum powders prior to the processes of agglomerating and annealing.

To handle the powders with ease, the tantalum powders doped with phosphorus were classified into 150-mesh and agglomerated. The agglomerated tantalum powders were charged in the tantalum vessel to be processed by an annealing at 1,250° C. for 30 minutes under a vacuum level of 10⁻⁵ torr or more in the vacuum sintering furnace. The annealed powders were cooled at room temperature and passivated with an Ar-1% O₂ mixed gas for more than two hours. Subsequently, the passivated powders pulled out from the furnace were subjected to a deoxidizing process at 900° C. for four hours after mixing with 325-mesh or less of Mg powders 1% to 5% uniformly to remove residual oxygen in the powders. Then, the deoxidized powders were subjected to a leaching process by adding sulphuric acid H₂SO₄ solution at 40° C. to 80° C. thereto till Mg ions were removed. Subsequently, hydrogen peroxide H₂O₂ solution of 1% to 10% was added to prevent the tantalum powders from reacting with hydrogen. The powders passing the leaching process were washed to be neutralized with purified water heated to 40° C. to 80° C. and vacuum dried.

To measure electrostatic capacitor, the dried powders were molded with a tantalum wire in a rectangular mold to have a density of 4.5 to 5.0 g/cm³. The molded pellet was sintered using the vacuum sintering furnace at 1,200° C. for 30 minutes. The sintered pellet was transformed in 0.1 vol % of phosphorus H₃PO₄ solution heated to 60° C. by applying a direct voltage of 40 V to form an amorphous tantalum oxide layer. The results of measuring impurity contents and electric characteristics for the tantalum powders prepared were shown in Tables 1 and 2 below.

Embodiment 3

Niobium pentoxides Nb₂O₅ having a content of metal impurities of 150 ppm or less and a carbon content of 10 ppm or less were used. The molten salt electrolytic reducing reactor of FIG. 1 was charged with 1 kg of LiCl, 33.7 g of Li₂O and 75 g of MgCl₂. 100 g of Nb₂O₅ having an average grain size of 0.1 μm together with steel wool were put into the cathode basket and heated at 650° C. A current was applied in order to keep a voltage of −2.5 V to −3.0 V, over the decomposition potential of Li₂O for two hours. And, a current was applied in order to keep a voltage of −2.8 V to −3.0 V, over the decomposition potential of MgCl₂ for three hours. Here, temperature variations of the thermocouple inserted therein were observed to determine to change the voltage. That is, if a rapid increase of the temperature was observed, the voltage was decreased. After terminating the reaction, the resultant product was separated from the molten salt so as to facilitate the extraction of the product after the molten salt was cooled. The extracted product was cooled at room temperature and coarsely crushed so as to shorten the time required for the steps of leaching and washing. During the leaching step, an inorganic acid of 20%, particularly, a sulphuric acid H₂SO₄ solution was added and applied heat of 40° C. to 80° C. thereto. Subsequently, hydrogen peroxide H₂O₂ solution of 1% to 10% was added to prevent the niobium metal powders from reacting with hydrogen.

The niobium powders for electrolytic capacitors were doped with a sintering inhibitor such as nitrogen, phosphorus, boron, sulfur, etc., to prevent over-sintering in the step of sintering. In the present invention, the sintering inhibitor in the form of NH₄H₂PO₄ was mixed with phosphorus to be 100 ppm in proportion to the content of niobium powders and added to the niobium powders prior to the processes of agglomerating and annealing.

To handle the powders with ease, the niobium powders doped with phosphorus were classified into 150-mesh and agglomerated. The agglomerated niobium powders were charged in the tantalum vessel to be processed by an annealing at 1,200° C. for 20 minutes under a vacuum level of 10⁻⁵ torr or more in the vacuum sintering furnace. The annealed powders were cooled at room temperature and passivated with an Ar-1% O₂ mixed gas for more than two hours. Subsequently, the passivated powders pulled out from the furnace were subjected to a deoxidizing process at 800° C. for four hours after mixing with 325-mesh or less of Mg powders 1% to 5% uniformly to remove residual oxygen in the powders. Then, the deoxidized powders were subjected to a leaching process by adding sulphuric acid H₂SO₄ solution at 40° C. to 80° C. thereto till Mg ions were removed. Subsequently, hydrogen peroxide H₂O₂ solution of 1% to 10% was added to prevent the niobium powders from reacting with hydrogen. The powders passing the leaching process were washed to be neutralized with purified water heated to 40° C. to 80° C. and vacuum dried. The dried powders had the form of agglomerations of 150 μm or less as shown in FIG. 8 and the first niobium grain has a mesh structure of 0.3 μm or so.

To fulfill an assessment of electrostatic capacitor, the dried powders were molded with a tantalum wire in a rectangular mold to have a density of 4.5 to 5.0 g/cm³. The molded pellet was sintered using the vacuum sintering furnace at 1,150° C. for 30 minutes. The sintered pellet was transformed in 0.1 vol % of phosphorus H₃PO₄ solution heated to 60° C. by applying a direct voltage of 40 V to form an amorphous niobium oxide layer. Assessed results of impurity contents and electric characteristics for the niobium powders prepared were shown in Tables 1 and 2 below.

TABLE 1 Impurities of End Tantalum and Niobium Powders (ppm) Embodiment O C N Fe Ni Cr K Na Ca Mg 1 (Tantalum) 3,5000 45 900 10 10 15 10 10 35 15 2 (Tantalum) 4,500 45 900 10 10 15 10 10 35 15 3 (Niobium) 5,600 50 950 10 10 15 10 10 40 15

TABLE 2 Electric Characteristics of Tantalum and Niobium Powders (Sintering at 1,200° C. for 30 min., 0.1 vol %-H₃PO₄, 40 V, 60° C.) DC Density of Leakage Electrostatic Molded Density of Current Capacity tan Pellet Sintered Embodiment NA/CV μFv/g δ % g/cm³ Pellet g/cm³ 1 (Tantalum) 0.32 76,000 45 4.7 5.5 2 (Tantalum) 0.42 86,000 50 4.56 5.2 3 (Niobium) 1.1 110,000 65 2.6 2.75

As described in detail above, according to the method for preparing tantalum Ta and Niobium Nb powders suitable for manufacturing capacitors in accordance with the present invention, the tantalum pentoxides Ta₂O₅ or the niobium pentoxides Nb₂O₅ are electrochemically indirect-reduced in sequence in the alkaline metal or alkaline earth metal halogen molten salt, in which alkaline metal oxides are dissolved, using the difference of reducing potentials of the alkaline metal oxide and the alkaline metal halogen compound. Accordingly, the method of the present invention has been developed to overcome the drawbacks, such as the explosive reaction, the dispersion of reaction heat, the second phased reaction and the like, caused by the conventional heavy metal reduction method via alkaline earth metals or via hydrides, and to control the grain sizes to be reproducible, thus preparing economically tantalum or niobium powders in high purity suitable for manufacturing capacitors.

As described in detail above, the present invention has been disclosed herein with reference to preferred embodiments, however, it will be understood by those of ordinary skill in the art that various changes in form and details may be made without departing from the spirit and scope of the present invention as set forth in the following claims. 

1. A method for preparing tantalum or niobium powders used for manufacturing capacitors in an electrolytic reducing reactor including an anode, a cathode and a molten salt, the method comprising: obtaining a tantalum or niobium oxide, expressed by Ta₂O_((5-y)) or Nb₂O_((5-y)) where y=2.5 to 4.5, from a tantalum pentoxide Ta₂O₅ or a niobium pentoxide Nb₂O₅ generated partially by an alkaline metal electrolytically reduced via a first electrolytic reducing reaction that reduces an alkaline metal oxide from a molten salt comprising at least one metal halogen compound, selected from the group consisting of alkaline metal and alkaline earth metal, and an alkaline metal oxide on the cathode; and preparing a tantalum or niobium powder by a first electrolytic reducing reaction that reduces at least one metal halogen compound selected from the group consisting of the alkaline metal oxide and the alkaline earth metal on the cathode and by a second reducing reaction with the tantalum or niobium oxide, represented by Ta₂O_((5-y)) or Nb₂O_((5-y)) where y=2.5 to 4.5.
 2. The method for preparing tantalum or niobium powders used for manufacturing capacitors as recited in claim 1 further comprising: leaching and washing the tantalum or niobium powder, obtained during the second reducing reaction, in an inorganic acid.
 3. The method for preparing tantalum or niobium powders used for manufacturing capacitors as recited in claim 1, wherein the tantalum or niobium powder obtained during the second reducing reaction has a mesh structure.
 4. The method for preparing tantalum or niobium powders used for manufacturing capacitors as recited in claim 1, wherein the tantalum pentoxide Ta₂O₅ or the niobium pentoxide Nb₂O₅ is used in the form of powder or porous pellet.
 5. The method for preparing tantalum or niobium powders used for manufacturing capacitors as recited in claim 4, wherein the tantalum pentoxide Ta₂O₅ or the niobium pentoxide Nb₂O₅ porous pellet is prepared by molding the tantalum pentoxide Ta₂O₅ or the niobium pentoxide Nb₂O₅ powders to form a molded pellet and sintering the molded pellet.
 6. The method for preparing tantalum or niobium powders used for manufacturing capacitors as recited in claim 4 or 5, wherein the tantalum pentoxide Ta₂O₅ or the niobium pentoxide Nb₂O₅ powders have a content of metal impurities of 150 ppm or less and a carbon content of 50 ppm or less.
 7. The method for preparing tantalum or niobium powders used for manufacturing capacitors as recited in claim 4, wherein the tantalum pentoxide Ta₂O₅ or the niobium pentoxide Nb₂O₅ in the form of powder is charged into a porous cathode basket.
 8. The method for preparing tantalum or niobium powders used for manufacturing capacitors as recited in claim 4, wherein a single type cathode including a high-conductive conductor and a non-conductive ceramic filter having a porosity of 10 to 60% and charged with Ta₂O₅ or Nb₂O₅ powders is used as a cathode.
 9. The method for preparing tantalum or niobium powders used for manufacturing capacitors as recited in claim 8, wherein the high-conductive conductor is made of one selected from the group consisting of SUS, copper, titanium and tantalum.
 10. The method for preparing tantalum or niobium powders used for manufacturing capacitors as recited in claim 8, wherein the non-conductive ceramic filter is made of one selected from the group consisting of magnesia, alumina, magnesium aluminate and zirconia.
 11. The method for preparing tantalum or niobium powders used for manufacturing capacitors as recited in claim 4, wherein the Ta₂O₅ or Nb₂O₅ porous pellet has a plurality of perforations.
 12. The method for preparing tantalum or niobium powders used for manufacturing capacitors as recited in claim 1, wherein reduction temperatures of Ta₂O₅ or Nb₂O₅ is controlled by an applied voltage.
 13. The method for preparing tantalum or niobium powders used for manufacturing capacitors as recited in claim 1, wherein platinum, pyro-carbon, or Fe₃O₄ is used as an anode.
 14. A method for preparing tantalum or niobium powders used for manufacturing capacitors in an electrolytic reducing reactor including an anode, a cathode and a molten salt, wherein the molten salt is composed of an alkaline metal halogen compound and an alkaline earth metal halogen, the alkaline earth metal halogen compound is first electrolytically reduced on the cathode, the electrolytically reduced alkaline earth metals reduce tantalum pentoxide Ta₂O₅ or niobium pentoxide Nb₂O₅ indirectly, and local sinterings are induced via controlling potentials. 